1. Field of the Invention
The present invention relates to optical control devices, and, in particular, to such devices of an integrated type or a waveguide type having a buffer layer.
2. Description of the Related Art
In the field of optical control devices, there exists optical switches for switching optical paths in an optical communication network, optical modulators for modulating an intensity or a phase of a light beam transmitted through the waveguide or waveguides, optical filters for filtering a light component of a particular wavelength, and others.
In the prior art, an optical control device of a waveguide type or an integrated type is known which comprises a light-transparent dielectric substrate with an electrooptic effect such as LiNbO.sub.3 or LiTaO.sub.3, a waveguide or waveguides formed in a main surface of the substrate, and electrodes formed on the main surface for controlling a light beam transmitted through the waveguide or waveguides by an electric driving voltage signal applied to the electrodes. The waveguide is formed by diffusing metallic ions into the main surface, typically, in-diffusion of Ti ions. The electrode is formed on the main surface in form of a thin metallic film such as Al, Au or other metals by the CVD method or other thin film forming methods.
There is known in the art such an optical control device of a type which has two waveguides at least partially parallel with each other and close to each other, and three or two electrodes disposed along the waveguides, one being disposed at an inside region between the two waveguides, the other at outside regions of the two waveguides.
In a known optical control device of the type described above, the electrodes are directly formed on the main surface of the substrate. In another known device, a so-called buffer layer is formed on the main surface of the substrate and the electrodes are deposited on the buffer layer.
The buffer layer is used for preventing a light beam transmitted through the waveguides from being absorbed by the electrodes and for enabling to adjust transmission velocity of a microwave through the electrodes by selection of a thickness and/or dielectric constant of the buffer layer to thereby facilitate the high speed operation of the device. The buffer layer is made of a dielectric and light-transparent material which has a refractive index smaller than that of the substrate and a relatively low dielectric constant. Typically, SiO.sub.2 film is used for the buffer layer.
However, it is known in the art that the use of the buffer layer causes problems of a so-called DC drift and an increased driving voltage.
The DC drift is a phenomenon in which a light output changes or shifts during operation by use of a DC voltage or a low frequency voltage applied to the electrodes. This phenomenon is considered to be caused by impurity ions in the buffer layer moving in response to the electric field generated by the voltage applied to the electrodes. Those impurities are believed to be Na, K and other ions taken into the buffer layer from the environment or Li ions from the substrate of LiNaO.sub.3 and LiTaO.sub.3.
Since a driving voltage applied to the electrodes is dropped at the buffer layer having the relatively low dielectric constant, the driving voltage must unfortunately be a relatively high voltage.
Further, it is known in the art that the optical control device suffers from a so-called temperature drift in which its operating voltage changes by temperature variation. This is because the substrate such as LiNbO.sub.3 or LiTaO.sub.3 has the pyroelectric effect so that localization of electric charges is caused by the temperature variation.